1. Field of the Invention
The invention relates to a master cylinder for a hydraulically actuated clutch or a brake system in a motor vehicle.
2. Description of the Related Art
A master cylinder for hydraulically actuating a clutch or brake system is known from, for example, U.S. Pat. No. 5,335,585. In this reference, the master cylinder is activated by the brake or clutch pedal, which is connected to the piston rod of the cylinder. The hydraulic pressure generated in the master cylinder is transmitted through a hydraulic fluid-filled system of lines to a slave cylinder, where it has the effect of displacing the working piston and thus of actuating, for example, a clutch-release mechanism or a brake.
The master cylinder disclosed in U.S. Pat. No. 5,335,585 has a piston unit with a piston rod. A ball-shaped head at the end of the piston rod facing the pressure space is surrounded by a spherical shell part arranged at an end of the ball-shaped head and by two spherical half shell segments, which are arranged radially with respect to each other and which grip opposing semicircular areas of the ball-shaped head. The spherical half shell segments have radially inward-pointing projections at ends facing the pressure space, which engage in a groove in a neck formed on the piston. Except for the head of the piston facing the pressure space, the outside wall of the piston is formed by a piston shaft sleeve, which radially surrounds the individual parts and holds them together. In this way, the piston and the piston rod are connected to each other positively and essentially without play but still with the ability to swivel with respect to each other.
The piston design described above is very expensive because of the large number of parts. More specifically, three support elements are required to support the ball-shaped head of the piston rod. Each of these elements must be produced with high precision and mounted precisely in the piston. Another disadvantage of this design is that it is not very stiff. A tensile force introduced via the piston rod causes the spherical half shell segments to spread radially apart. Because these segments rest directly on the relatively thin-walled piston shaft sleeve, the sleeve itself can also expand radially, so that ultimately the sliding properties of the piston in the cylinder can be negatively affected. As a result of this expansion of the spherical half shell elements, the support of the ball-shaped head gradually and irreversibly loosens over the course of time, as a result of which undesirable gaps which allow play can develop. It is also known that, for example, 1 millimeter of play in a master cylinder can cause a dead travel of approximately 5-6 millimeters at the clutch pedal, the exact distance depending on the lever ratio.
Another master cylinder with a piston and an attached piston rod which can swivel with respect to the piston is described in U.S. Pat. No. 5,794,512, in which a piston shaft sleeve designed with a bottom accepts two retaining shells for the ball-shaped head of the piston rod. The retaining shells are arranged radially with respect to each other. On a side facing away from the bottom of the piston, the retaining shells have semicircular recesses, which positively retain the ball-shaped head. The retaining shells are also provided with slots in their end surfaces. The slots forms several segments which grip the ball while also allowing it to swivel. The segments have at their ends, radially inward-pointing snap-in projections which are surrounded by a free space. When the ball-shaped head is snapped into its bearing seat, the segments are thus able to move radially outward into the free space. To prevent the ball-shaped head from shifting in an unwanted manner, the snap-in projections are provided on the side facing the ball with a sharp-edged transition, on which an undercut is provided.
A disadvantage of this design is that under alternating compressive/tensile forces on the piston rod, a certain amount of dead travel caused by intermediate play may occur. Especially in the case of tensile forces, the area of the ball-shaped head adjacent to the piston rod may serve as a ramp for the segments, which can then escape radially to the outside despite their sharp-edged snap-in projections.